1. Field of the Invention
This invention relates to a recording method, a photo addressable recording medium used for it, a display device and a recording device.
2. Description of the Related Art
In recent years, a photo addressable spatial light modulating device having a combination of a photoconductive switching element and a display element has been developed and put into practical use as a light valve in a projector and the like, and in addition, as mentioned in “Liquid Crystal Spatial Light Modulator and Optical Information Processing”, EKISHO, Vol. 2, No. 1, 1998, pp. 3–11, its possibility in the field of optical information processing has been discussed.
The photo addressable spatial light modulating device is formed by at a least photo addressing unit, an optical switching element and a display device, and adapted to apply designated voltage between the optical switching element and the display element and simultaneously vary the impedance of the photoconductive switching element according to the amount of received light to control the voltage applied to the display element and drive the display element, thereby displaying an image. Further, attention has been focussed on a photo addressable recording medium, in which an element having a memory property is used as the display element of the photo addressable spatial light modulating device to be separable from the writing unit.
As these elements capable of controlling voltage or current according to the amount of received light, in addition to a photo diode used in CCD and an amorphous silicon element used in a contact type image sensor, for an organic photoreceptor element, a two-layer functional separation type optical switching structure used in Elgraphy and a three-layer functional separation type organic optical switching element 10 have been examined. This organic photosensitive element has the advantages that it is applicable to an AC drive liquid crystal element or the like and that it is also applicable to a flexible substrate such as a PET film because high temperature heat treatment is not required.
As a display control element of a photo addressable medium, examination has been made on liquid crystal display elements such as nematic liquid crystal which is dispersed in polymer to have a memory property, cholesteric liquid crystal, ferroelectric liquid crystal and elements formed by high-polymer dispersing these liquid crystal or encapsulating the same, or an element formed by encapsulating a field rotary element, a toner display element, an electrophoretic element and the like.
Writing to such photo addressable medium is normally performed as follows. During irradiation with light, resistance of an optical switching element is lowered due to photoconductivity, so that most of voltage applied to the medium is divided to a display element part. At this time, to turn on a display, voltage above the threshold is applied to turn on a display. On the other hand, during non-irradiation with light, resistance of the optical switching element is increased, most of the applied voltage is divided to the optical switching element and the applied voltage to the display element drops below the threshold, so that the display is turned off.
The light irradiation area and the non-irradiation area are controlled to display an image.
The ordinary photo addressing element, as described, has the problem that as the variation of the resistance component due to light irradiation is large, high sensitivity cannot be obtained. It is because, in order to control the on-off state of the display, normally the resistance during irradiation should be 1/100 to 1/1000 as compared with the resistance during non-irradiation. For example, Fukushima et al. have proposed that the resistivity of a-Si film which is an optical switching element is lowered from 10E11 Ωcm to 10E8 Ωcm by irradiation of 1 mw/cm2 on pp 6859–6868 “Ferroelectric liquid-crystal spatial light modulator achieving bipolar image operation and cascadability”, APPLIED OPTICS '92 Vol. 31, No. 32, 1992, pp. 6859–6868. It is a matter of course that the display may be controlled by reducing the resistance value to 1/10, but a margin is small. Further, in the case of the display element or the optical switching element, since the time constant is several tens of msec to as large as several seconds, it requires time for setting until reaching a resistance divided value. When rewriting in a shorter time, the applied voltage becomes equal to or lower than the dividing proportion, and the margin becomes smaller so that it is difficult to control the display.
Consequently, the writing frequency is lowered to several tens of Hz and to several Hz in some cases, and further in the case of cholesteric liquid crystal or an element like a field rotary element, there is the problem that since the driving voltage is high and it is necessary to supply low-frequency/high driving voltage as a power supply, a large, high-cost power supply is needed.
Further, the display element having memory function used in recording has the problem that coloration is difficult. In coloration, in the case of using a ferroelectric liquid crystal element, for example, a display is made using a color filter by matrix drive, but in the case of recording, the problem is that there is no accuracy of addressing, so it is difficult to secure the accuracy of radiating a desired amount of light to a designated position of the filter. Of course, there are other ways of coloration, but none of them is put into practical use because of their own problems.